Solid oxide fuel cells (SOFCs) are used to generate electrical energy through electrochemical reactions between air and hydrocarbon fuel gas to produce a flow of electrons in an external circuit. Generators based on SOFCs offer a clean and highly efficient approach for electrochemical generation of electricity. Conventional solid oxide fuel cells are disclosed in U.S. Pat. Nos. 4,395,469 to Isenberg, 4,476,196 to Poppel et al., 4,476,198 to Ackerman, et al., 4,490,444 to Isenberg, 4,562,124 to Ruka, 4,751,152 to Zymboly, 4,767,518 to Maskalick, 4,888,254 to Reichner, 5,106,706 to Singh, et al., 5,108,850 to Carlson, et al., 5,277,995 to Ruka, et al. and 5,342,704 to Vasilow, et al. Each of these patents is incorporated herein by reference.
The air electrodes or cathodes of conventional solid oxide fuel cells typically have porosities of from about 20 to 40 percent, and also have good electrical conductivities. The air electrodes are usually comprised of oxides having a perovskite-like crystal structure (ABO.sub.3), such as LaMnO.sub.3 wherein the La occupies the A-site and the Mn occupies the B-site. In addition to doped LaMnO.sub.3 air electrodes, SOFCs typically consist of a yttria stabilized ZrO.sub.2 electrolyte, a doped LaCrO.sub.3 interconnection, and a Ni--ZrO.sub.2 cermet fuel electrode or anode.
Important properties for the components of SOFCs between room temperature and 1,000.degree. C. include chemical compatibility between each of the components, chemical stability of the air electrode in air, chemical stability of the fuel electrode in a reducing fuel atmosphere, and chemical stability of the interconnection in both oxidizing and reducing atmospheres, as well as a number of physical properties described below.
One air electrode material currently in use has a nominal composition La.sub.0.7 Ca.sub.0.2 Ce.sub.0.105 Mn.sub.0.94 Cr.sub.0.04 Ni.sub.0.02 O.sub.3. This material is an improvement over previously used material of the formula La.sub.0.8 Ca.sub.0.2 MnO.sub.3, in that it shows much less shrinkage when thermal cycled between 25 and 1,000.degree. C. However, it would be desirable to have a composition with a closer match of thermal expansion to the zirconia electrolyte, an even lower thermal cycle shrinkage, a thermal expansion which shows little or no change with time, and no perceptible phase change in the fabrication and operational temperature ranges of the SOFC.